1. Field of the Invention
This invention relates to woven fabrics of polyester fibrous yarns which will be found highly suitable as backing members in the manufacture of coated abrasive material and to their manner of manufacture as well as the coated abrasive material manufactured therefrom.
2. Description of the Prior Art
Coated abrasive material, or as it is more commonly known "sandpaper", is made of a flexible backing member to which is applied an adhesive layer and abrasive grain.
The flexible backing member of the coated abrasive material is, in general, a woven fabric of cellulosic yarn, i.e., cotton staple yarn; however, the prior art has also disclosed the use of yarns of man-made fibers such as nylon, polyester, polypropylene, polyethylene, and glass. Representative of this latter prior art are U.S. Pat. Nos. 2,712,987; and 3,316,072; and Canadian Pat. Nos. 676,601 to Hansen, Pemrick, and Sprague and No. 744,667 to Pemrick and Gladstone, Pemrick being one of the inventors named herein. Others include U.S. Pat. Nos. 2,672,715; 2,740,239; 3,246,969; and 3,487,593.
As received from the mill, or cloth manufacturer, cotton fabric backing members are too permeable and the greige fabric possesses too much inherent longitudinal stretch to be suitable, as is, as a backing member for coated abrasive manufacture. Because of this, the fabric is subjected to a "cloth finishing" operation whereby the fabric is subjected to a number of different separate and distinct operations. This includes dyeing the greige fabric brown or blue, or some other desirable color for identification purposes, followed by pull-down or longitudinal stretching of the fabric to align the fibers and to impart to the fabric some desired dimensional stability. Afterwards, the fabric may be subjected to one or more backing filling operations, followed by application of a back size and front size composition. To the front side of the fabric may be further applied another coat providing for improved adhesion with the maker adhesive to be applied.
These various and sundry cloth finishing steps have been considered necessary in the past to provide, among other things, suitable flexibility in the backing member and dimensional stability in the coated abrasive material. The back fill and front size operations have been considered critical where a heat-hardened resinous maker adhesive is to be utilized in coated abrasive manufacture; otherwise, the adhesive will penetrate into the cotton backing member and embrittle the cellulosic yarns.
It was to eliminate or at least reduce the number of distinct operations involved in the cloth finishing of cotton fabric backing members, as well as, among other things, to provide coated abrasive material of improved flexibility and durability, that first led to the invention disclosed in Canadian Pat. No. 676,601. Therein, as disclosed by the patentees, woven fabric backing members of isotactic polypropylene, linear and branched polyethylene, and polyester yarns, of either staple fibers or continuous filaments, result in minimum cloth treatment, compared to cotton fabrics, to prepare the fabric for use as a coated abrasive backing member.
As disclosed in Canadian Pat. No. 676,601, the fabric therein can be dimensionally stabilized by heating it to a temperature below but near the melting point of the yarn, while the fabric is tentered or relaxed. This causes some shrinkage in the fabric, resulting in a denser fabric, and heat sets the fabric. On the other hand, where the fabric is tentered during heating, the fabric is heat set in its original dimension.
To avoid the occurrence of excessive elongation where a woven fabric is to be used as a backing member to manufacture abrasive material, it has been customary, to build into the woven fabric and therefore into the coated abrasive material, the various strength elongation, and other characteristics desired. This has been accomplished in the past by using yarns of certain desired fibrous materials, e.g., cotton, sizes, and fabrics of certain construction, i.e., thread count, yarn number, weave patterns, etc.
Standard fabric constructions result in fabric cover factors, in the greige fabric, in the range of 80% to 95%. In other words, the openness or air space in the fabric is of the order of 5% to 20%. The percent cover factor is defined as 100 - % air space; ##EQU1## After pull-down during cloth finishing, the cover is from about 85 to 96%.
The use of man-made fiber fabrics heretofore as backing members for coated abrasive material, except for polynosic rayon, has met with only very limited success. Among other things, especially with polyester fabrics, adhesion between the backing member and the maker adhesive has not been completely satisfactory. Conventional maker adhesives of phenolic resins do not wet polyester fibers; therefore no chemical bonding occurs between the maker adhesive and backing member. Adhesion is solely mechanical and this has been found to be somewhat limited. This results in shedding of the abrasive grain when the abrasive material is still useful.
One of the major problems, however, in the past deterring the use of man-made fiber fabrics results from the fact that, e.g., available polyester fabrics have lacked suitable dimensional stability. This is particularly true even where these fabrics are dimensionally stabilized as mentioned in Canadian Pat. No. 676,601, particularly where the coated abrasive material is used in the manufacture of larger abrasive belts such as used in plywood sanding or in applications where the coated abrasive belt is subjected to relatively high tensile forces.
A fabric can be somewhat dimensionally stabilized by either stretching the fabric, and this is disclosed in the prior art, or already stretched yarns can be used in weaving the fabric. This is the manner of providing a dimensionally stabilized coated abrasive backing member of nylon film disclosed in U.S. Pat. No. 2,712,987, earlier mentioned in the description of the prior art. U.S. Pat. No. 3,517,425 disclosed heat stretching tire fabric to reduce its elongation in use.
Other means of providing some dimensional stability to fabrics involves application of coating materials to the fabric. Such a method of stabilizing a fabric is disclosed in U.S. Pat. No. 2,977,665 wherein a polymeric material such as a mixture of thermosetting and thermoplastic resins is applied to the fabric, and while the fabric is stretched, the resinous material is cured.
One might readily assume from the above prior art, that an obvious solution to undesired fabric elongation in a polyester fabric backing member in coated abrasive material, during usage of the abrasive material, would be to stretch the woven greige fabric intended to be used as a backing member even more than has been done in the past, particularly with respect to cotton fabrics. However, this approach has been found to accomplish little in the fabric constructions conventionally used except to finally result in a torn fabric. The yarns have been found to become so jammed up in the fabric being subjected to high pull-down forces to reduce elongation to the desired level that further stress on the fabric merely results in tearing.
When a backing member of polyester yarns is provided of the same cloth construction found most suitable, in cotton drills and jeans, the woven fabric has been found to have elongation at levels ranging from 25% to 85% in the warp direction and up to 135% in the fill. About 10-20% of the total elongation can be removed by conventional mechanical pull-down techniques; however, beyond this the fabric jams, i.e., during pull-down, the warp and fill yarns move closer and closer together thereby increasing percent fabric cover. When jamming occurs, increased tension only results in tearing.
Part of the remaining elongation can be removed by means of chemical stabilization, i.e., application of resinous coatings; however, this can be accomplished only at the expense of reduced physical characteristics such as tensile and tear strength. This of course is undesirable.